In pressurized-water nuclear power stations, steam is produced by means of steam generators. These steam generators receive primary pressurized water at a high temperature obtained by circulating the primary water between the fuel elements placed in the vessel of the reactor.
The pressurized primary water circulates in a large number of heat-exchanger tubes of the steam generator. These U-shaped tubes have a small cross-section and a great length. When the preheated water of the secondary circuit comes into contact with the tubes, it evaporates to provide steam intended for feeding the turbines which will themselves cause the rotation of an alternator for generating electricity.
The heat-exchanger tubes of the steam generators form a highly sensitive point, and because of their small thickness they constitute functional reactor elements of major importance which have to be monitored constantly.
Thus, during refuelling shutdowns, these tubes are checked regularly to ensure that they are intact, because the leakage or breakage of one or more of them would require immediate shutdown of the reactor, in order to carry out the work and repairs necessary for putting the relevant tube or tubes out of action.
When faults exceeding the criteria of acceptability are detected in the tubes, it is necessary for them to be repaired. A method currently adopted involves covering the fault by means of a sleeve, so as to ensure sealing between the primary circuit and the secondary circuit and to allow the tube to continue to perform its funtion.
The sleeve is fastened by mechanical positioning (expansion) and then by introducing into the tube a welding tool holder comprising, for example, a laser welding appliance. It is therefore necessary to know the position of the end of the sleeve accurately so that the tool holder can be positioned correctly.